The invention relates generally to the field of load management for electrical utilities and is concerned primarily with controlling peak power demand.
The design capacity of electric utility generating and distribution equipment should be sized on the basis of coincidental load factors. The generating capacity of the utility must be greater than the maximum demand for electricity experienced during the year. Electrical demand, however, is unevenly distributed throughout the year; and to meet seasonal demands, the electric utilities are faced with building additional generating stations and distribution facilities to meet a peak demand encountered on only a few days caused by electric air conditioning on the hottest summer day or by heating on the coldest day of the year. The rest of the year much of this equipment stands idle or operates unecomonically at low load factors. Presently the nation's utilities have an average equipment utilization factor of 40 percent in a 24-hour period. Moreover, the available peak generating capacity is reduced by removal of equipment for maintenance procedures.
Unable to keep pace with the rapid growth of electrical peak power loads, power companies have found the total load at certain hours of particular days of the year running within 10 percent of maximum capacity, well beyond the safer margin of 20 percent which is normally required. Running this close to capacity is hazardous since failure of any generating or distribution equipment will result in a substantial overload, often with disastrous results such as brown outs or widespread interruption of service. In many instances utilities are forced to purchase power from neighboring utilities at great costs, because of the cost and substantial lead time required to build extra generating stations and transmission equipment which, due to coincidence peaks, would remain idle much of the time.
Attention is being devoted to schemes for trimming or "shaving" the demand peaks of the seasonal and monthly variety so that maximum generating capacity will be more in line with the average demand throughout the year to promote economical operation and safer operating margins on peak days. Disconnecting or "shedding" certain customer loads on a priority basis is one means of establishing reasonable capacity margins. All utilities may have various types of loads, some of which are what may be classified as controllable or interruptable loads, for example, electric hydronic heating, snow melting, warehouse heating and cooling, marginal lighting, etc. Unlike these seasonally varying loads, however, domestic hot water, for example, represents a controllable base load which is present 24 hours a day, 12 months a year, and may be interrupted without inconvenience to the customer.
Selective load dropping according to priority schedules is discussed in U.S. Pat. No. 3,359,551 to Dennison; Brinks, "Controlling High-Capacity Power Demand", Electrical Construction and Maintenance, January 1971, page 74; and Bailey, "Selective Energy Use - Proposing a New Dimension in Electric Power Control", Control Engineering, 1972. These references note that residential commercial or industrial loads may be turned off remotely, for example by radio signals transmitted from a central station in the event of a power shortage. U.S. Pat. Nos. 2,641,716 to Hyer et al and 3,540,030 to Hartz, and the Dennison patent, supra, specifically discuss shedding of conventional hot water heater loads with controlled reconnection according to fixed schedules.
While hot water heater loads used by customers for domestic water, process water or space heating may represent the best form of controllable nonessential electrical load for peakshaving, customer acceptance, reliable service and increased cost remain the most serious problems. Depending on the nature of the use, the time of day and the reserve capacity, some hot water heater loads are less likely to cause service interruption than others if removed from the line for a given period of time. The prior art systems failed in particular to distinguish on a rational day-to-day basis between these different classes of hot water heating loads and varying costs of power. In addition, sudden interruption of loads and subsequent uncontrolled reconnection may result in a rate penalty for the user. Indiscriminate reconnection of a number of controlled loads after a period of disconnection can also produce a secondary demand peak at the power company.